Email subscription params in request body

[Jaskon]

According to issue #651

[codecov[bot]]

Codecov Report

Patch coverage: 66.66% and no project coverage change.

Comparison is base (07667c8) 58.63% compared to head (d4b873b) 58.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1645 +/- ## ======================================= Coverage 58.63% 58.63% ======================================= Files 128 128 Lines 2877 2877 Branches 730 737 +7 ======================================= Hits 1687 1687 Misses 1065 1065 Partials 125 125 ``` | [Impacted Files](https://app.codecov.io/gh/umputun/remark42/pull/1645?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun) | Coverage Δ | | |---|---|---| | [frontend/apps/remark42/app/common/api.ts](https://app.codecov.io/gh/umputun/remark42/pull/1645?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvYXBwcy9yZW1hcms0Mi9hcHAvY29tbW9uL2FwaS50cw==) | `72.09% <50.00%> (ø)` | | | [frontend/apps/remark42/app/common/fetcher.ts](https://app.codecov.io/gh/umputun/remark42/pull/1645?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvYXBwcy9yZW1hcms0Mi9hcHAvY29tbW9uL2ZldGNoZXIudHM=) | `94.82% <100.00%> (ø)` | |

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